The present invention relates to, for example, a CMOS image sensor suitable for a solid image pickup device such as a digital camera.
A device having a plurality of photodiodes (hereinafter, abbreviated to PDs) arranged in a matrix is known as a conventional CMOS image sensor. Each of these PDs constitutes one pixel respectively. In the CMOS image sensor, output circuits are arranged in a matrix to fill up gaps between the PDs. The matrix of the output circuits is a two-dimensional matrix having repetitive pitches in row and column direction, which are n and m times as long as those in the matrix of the PDs. Each of the output circuits commonly reads two PDs arranged in the row.
Furthermore, a CMOS image sensor having output circuits, each of which is so designed as to commonly read four PDs arranged in the column to increase the integration density (refer to U.S. Pat. No. 6,091,449).
Meanwhile, in such a conventional CMOS image sensor, high integration circuit is obtained because a signal of two or four PDs arranged in the column is read by a common output circuit. The high integration is effectively realized in the column direction, that is, in the vertical direction of the screen. However, the high integration is difficult to realize in the horizontal direction, because the output circuits are arranged in the row direction at each column, where pixel signal cannot be detected. Particularly, when the CMOS image sensor is applied to a digital camera, it is necessary to make the pixel pitches equal to each other both in the horizontal and vertical directions. Thus, the high circuit integration only in the vertical direction is less advantageous in the layout of the integrated circuit.
Further, in the conventional sensor, the pixels and output circuits are horizontally arranged side by side, so that an opening for each pixel is vertically long, that is, it is rectangle having a vertical side longer than a horizontal side. Even in CCD area sensors, the opening for each pixel is also vertically long because there is a CCD register arranged side by side with each pixel in the horizontal direction. On the other hand, the aspect ratio of the optical system of the camera general used is 3:4, which is rectangle having a horizontal side longer than a vertical side. Therefore, taking a light entering through a lens to the photosensitive surface of the sensor into consideration, in particular, the lights entering into the pixels located at peripheral area of the photosensitive surface, the light enters more slantwise to the pixel located in the horizontally end portion than to the pixel located in the vertically end portion. Therefore, an output level obtained from the pixels located in the peripheral area is generally lower than that obtained from the pixels located in the central area, which is so-called shading. Vertically long openings for pixels are more easily affected by the shading than horizontally long openings.
Furthermore, in the conventional CMOS image sensor, a reading gate and a floating junction shared by the upper and lower pixels are installed between the pixels in the vertical direction. In application to a digital camera or a camera of a portable telephone, it is necessary to arrange a plurality of pixels in a square matrix. In addition, the openings for pixels are desirable to be arranged at an equal interval. Therefore, the intervals between the pixels in the vertical direction where the reading gate is not installed must be equal to the intervals between the pixels where the reading gate is installed and thus the space between the pixels in the vertical direction could not be used effectively.